For a number of years now, optical media such as CD-ROM's (compact diskread only memory) and CD-DA's (compact disk-digital audio) have been used as mass storage devices for storing large amounts of information. Traditionally, information is stored onto a CD in the form of a series of pits and lands. A pit is a physical indention in the medium, whereas a land is a flat area in which there is no indention. Pits and lands are typically imposed onto a CD using a mechanical stamping mechanism. Because they are at different elevations, pits and lands reflect light differently. This difference in reflection is exploited at read time to extract information from the CD. More specifically, to read information from a CD, a light source, such as a laser, is directed at the surface of the CD. As this light source scans the various pits and lands, the light is reflected, and the reflections are capture by a detection mechanism. Because pits and lands reflect light differently, the reflection from a pit will differ from the reflection from a land. This difference is detected by the detection mechanism. By detecting the differences in reflections, the detection mechanism is able to discern the pits from the lands, which in turn makes it possible to extract the stored information (e.g. the 1's and 0's) from the CD.
While traditional CD technology has been effective for storing large amounts of information onto relatively small CD's, it does have several shortcomings. The first is that a typical consumer cannot store information onto a CD. Because the pits and lands must be made by a sophisticated stamping mechanism, and because this mechanism is usually quite expensive, most consumers cannot afford the equipment necessary for storing information onto a CD. A second shortcoming is that information can be recorded onto a traditional CD only once. Once the pits and lands have been mechanically stamped onto the CD, they cannot be removed. Because of these two limitations, CD's have primarily been used in the past only by the entertainment and software industries as a medium for distributing their merchandise. CD's have traditionally not been used by consumers as a recording medium.
In recent years, however, new technologies have emerged that make CD's much more usable as recording media. These technologies include CD-R (compact disk-recordable) and CD-RW (compact-disk rewritable). With CD-R and CD-RW, information is imposed onto a CD not by stamping pits and lands but by burning in marks and spaces. A mark is an area which has been burned by a heat source, such as a laser, whereas a space is an area which has not been burned or just slightly burned. CD-R and CD-RW operate under the same general optical principles as traditional CD's. Basically, a mark reflects light differently than a space. This difference in reflection is detectable by a detection mechanism to discern the marks from the spaces. Thus, as with traditional CD's, information can be extracted by applying a light source, and by detecting the differences in reflections. That is where the similarity stops, however. Because information is imposed by burning marks and spaces as opposed to stamping pits and lands, all of the shortcomings associated with stamping are eliminated. Namely, no expensive stamping equipment is needed to record information. Instead, a relatively inexpensive burning mechanism (such as a laser) can be used. Also, because the marks and spaces, unlike pits and lands, are not physical marks, they can be erased (from certain special materials). As a result, a CD can now be rewritable. These and other aspects of CD-R and CD-RW enable CD's to be used by consumers as general recording media.
CD-R and CD-RW do not come without their shortcomings, however. One major shortcoming is that the current technology has problems controlling the precise dimensions of the marks and spaces that are imposed onto a CD. At times, a mark or a space is longer than it should be, and at other times, it is shorter than it needs to be. Because the dimensions of the marks and spaces determine the information stored on the CD, errors in these dimensions can lead to incorrect information being stored. Such errors can give rise to serious consequences. As a result, an improved writing control mechanism is needed to ensure accuracy of the writing process.